ON THE MORNING OF March 24, 1999, a truck carrying 12 tons of flour and nine tons of margarine entered the French side of the Mont Blanc Tunnel, the two-lane, 7.2-mile tube connecting France and Italy beneath the Mont Blanc massif in the Alps.

Smoke began coming from the area around the truck’s cab, and at 10:52 a.m. the smoke was detected by a tunnel smoke sensor. Command centers were located at either end of the tunnel to monitor traffic and conditions, with the help of cameras and sensors, and the sensor triggered an alarm in the command center on the French side; the sensor on the Italian side had been turned off due to a false alarm the day before. The French operator could not determine the cause of the alarm, though, and vehicles continued to enter the tunnel from both ends.

Seconds From Disaster - Tunnel Inferno

Via YouTube.com/user/DjRichard1994

Midway through the tunnel, the truck’s driver noticed the smoke and stopped. He got out of the cab to investigate, but before he could even grab his fire extinguisher the cab exploded. The driver began running toward the Italian end of the tunnel.

At 10:54, someone in the tunnel used an emergency phone in a rest area to alert personnel in the Italian command center, who immediately contacted their French counterparts. Both entrances were closed, but immediately behind the burning truck was a line of cars and trucks with nowhere to go. Thick smoke, pushed by air flowing from the Italian side, began to flow over and envelop the vehicles. Drivers coming from the Italian side saw the smoke ahead and backed up until they reached a rest area, where they turned around and fled to safety. Some of the trapped drivers in the tunnel left their vehicles and ran for safety, only to succumb to the smoke and fumes. Four minutes into the fire, firefighters stationed at the French entrance received the alarm; a four-man crew responded but was turned back by the growing fire. The smoke was so thick that they had been unable to see the vehicles behind the burning truck.
The intense fire soon involved those vehicles as well, and the tunnel became an inferno of exploding fuel tanks and impenetrable smoke. Temperatures were estimated to have reached more than 1,800 degrees F, hot enough to ignite the asphalt road surface. People died in their cars, their seatbelts still fastened. Others who ran and managed to reach the tunnel’s emergency refuges, which had a two-hour fire rating, fared little better; temperatures inside the refuges reached an estimated 700 degrees, killing the inhabitants. The fire burned for more than 50 hours.

When firefighters were able to re-enter the tunnel, they discovered the blackened remnants of 25 vehicles and the remains of 38 people. One firefighter was also killed in the blaze.